It is often desirable to bring together a stream of gas with a stream of liquid in order that a chemical or physical reaction or mass transfer may take place between the two streams. One particular example of such a condition is the contacting of the products of combustion arising from a flame with a liquid in order to transfer heat to the liquid. Other examples include the contacting of a cooling refrigerant gas with a liquid in order to cool the liquid or the absorption of carbon dioxide or hydrogen sulfide from acid gas by contacting with ethanolamines or gylcols.
When gases and liquids are to be contacted in order to precipitate a chemical or physical reaction, the rate at which the process proceeds depends upon the surface area over which such contacting occurs. In such cases, in order to obtain high rates it is desirable to maximize this contact surface area.
Direct contact heat transfer (DCHT) is the technique by which heating and heated materials are brought into intimate contact with each other without the presence of an intermediate heat transfer surface or barrier. One example of DCHT is in the heating of fluids, primarily water, by the direct contacting of the products of combustion with the liquid. Steam can be raised by spraying water into a stream of hot gases issuing from a burner. Alternatively, hot gas streams generated by combustion can be bubbled through a liquid using submerged combustion heaters to effect heat transfer. Maximization of contact surface area is a desirable objective in each of these cases.
In the case of submerged combustion heaters, the flame or combustion zone is separated from the fluid by a protective tube or cylinder which acts as a shroud or shield. Such shielding elements are often susceptible to severe scaling and corrosion. Similar problems are typical in the case of downhole steam generators for use in injecting heated water or steam into oil-bearing underground formations. The annular metallic sleeves which surround the flame and transport water to the exhaust gas zone in downhole or surface heaters suffer particularly from scaling problems.
When a flame is contained within a protective shroud the shroud may on occasion be raised to very high temperatures. Where cooling is not provided the shroud may become glowing hot or even white hot. Where metals are used in such circumstances, cooling is provided to limit rapid deterioration of the metal in the shroud. When cooling, usually in the form of circulating water, is applied to the exterior surface of the flame shroud, steep thermal gradients are formed within the shroud wall. This can lead to metal fatigue and failure of the mechanical integrity of the shroud wall.
The same corrosion and fatigue problems arise when unquenched combustion exhaust gases are introduced into contact columns where direct contact cooling occurs. To protect trays and other contact surfaces within such columns, it is often necessary to pre-cool combustion gases.